The healing of wounds is a complex process which is often further complicated by the presence of non-viable, necrotic tissue in the wound area. Debridement is the process of removing the non-viable tissue from a wound to prevent infection and facilitate healing.
Considerable efforts have been made to discover materials capable of distinguishing between viable and non-viable tissue. The discovery of materials which would digest devitalized tissue while not attacking viable tissue would make it possible to remove the devitalized tissue without surgery. It would be a beneficial therapeutic agent in virtually all disease processes where topically devitalized tissue needs to be removed from the viable organism such as burns, decubitus ulcers, pressure necroses, incisional, traumatic and pyogenic wounds, and ulcers secondary to peripheral vascular disease.
One area that has attracted considerable attention is the use of proteolytic enzymes and other chemicals to effect the early debridement of eschar tissue, resulting from burns. Such devitalized tissue is an excellent culture medium and the principal source of the septicemia which is the proximate cause of death in the majority of severely burned patients.
In burns, the devitalized tissue is referred to as eschar. Burn eschar is a complex mixture of dried blood, purulent exudates, and denatured proteins normally found in the epidermal and dermal skin layers. The denatured proteins found in eschar are primarily collagen, elastin, fibrin, hemoglobin, and other coagulated proteins.
Collagen comprises about 75% of the skin's dry weight and is the main constituent of the necrotic debris and of eschar. Strands of semi-viable, compromised collagen, whose protective mucopolysaccharide sheath has been damaged or destroyed, anchor the necrotic tissue to the wound surface. These strands, which appear as a layer of whitish, persistent microscopic fibrils, have been termed `collagen moss`; they must be fully eliminated in order for the necrotic material to be separated from its base. This complete debridement then permits development of granulation tissue.
For a proteolytic enzyme to be most useful as a debriding agent, particularly for burns, it is desirable for the protease to distinguish between viable and non-viable tissue; readily and thoroughly hydrolyze a wide variety of denatured proteins found in eschar; function at physiological pH and temperature; be compatible with adjunct therapies (e.g., cleansing agents, topical antibiotics); not interfere with normal wound healing or complicate skin grafting; and remain stable in various formulations and at a wide range of temperatures. A number of proteolytic enzyme preparations have been used as debriding agents with varying degrees of success.
Travase.RTM. ointment, which is a preparation containing proteolytic enzymes obtained from sterile filtrates of Bacillus subtilis, is another known enzymatic debriding agent (Garrett, Clinical Medicine (1969) 76:11-15) and U.S. Pat. No. 3,409,719. Crikelair (U.S. Pat. No. 4,276,281) describes the use of elastase, a serine protease derived from pancreas, as an enzymatic debridement agent. Klein et al. (U.S. Pat. No. 4,329,430) describe a proteolytic enzyme mixture derived from bromelin which is useful for the digestion, dissection and separation of non-viable, devitalized tissue. Schmitt (U.S. Pat. No. 3,983,209) teaches treating burns in animals by applying enzymes to a burn surface for debridement of eschar and necrotic tissue. The enzymes disclosed included papain, trypsin, lysozyme, streptokinase, fibrinolysin, pinguinain, Travase and bromelain in a specified hydrophobic polymer. Bioerosion over prolonged periods of time slowly released the proteolytic enzymes.
Merkel (U.S. Pat. No. 3,677,900) discloses that collagenases, especially those produced by a species of Vibrio, are useful as debriding agents. However, Merkel's collagenase is an enzyme capable of digesting native, undenatured collagen under physiological conditions of pH and temperature and is not inhibited by serum. The Merkel collagenase therefore is not likely to distinguish between viable and non-viable tissue. Collagenases also have a very narrow substrate specificity. Further, collagenase has been reported to cause significant damage to viable dermal tissue, causing reduced granulation tissue development and wound maturation (Hamit et al., Ann. Surg. (1960) 151:589).
Further, proteolytic enzymes have been speculated to be useful in prevention of surgical adhesions (Ellis, Br. J. Surg. (1982) 69:241-243). Adhesions are characterized by fibrinous formations which develop within a few hours of operational trauma and become organized by the ingrowth of fibroblasts and blood capillaries to form established fibrous adhesions. Intra-abdominal adhesions are almost inevitable after major abdominal surgery. These adhesions make reoperation on the abdomen a tedious dissection, with risk of visceral damage, and they may precipitate intestinal obstruction. Numerous protein preparations have been tried, with varying degrees of success, to prevent surgical adhesions including heparin, corticosteroids, antihistamines, plasmin, streptokinase, dextran, and tissue plasminogen activator (Doody et al., Fertil and Steril (1989) 51:509-512), and no agent has proven to be consistently effective.
While other proteolytic agents are known which have speculated debriding and anti-adhesion properties, many of these agents have been shown to be ineffective and cause local or systemic toxicity. None of the previous proteolytic agents have the superior characteristics of the present invention, including debridement properties and the ability to promote the wound healing process.